1. Field of the Invention
The present invention relates to a signal processing apparatus and an image pickup apparatus using an image pickup device and, more particularly, to a signal processing apparatus for reducing a foldover distortion and an image pickup apparatus provided with this signal processing apparatus.
2. Related Background Art
A conventional example will now be described with respect to an image pickup apparatus using an image pickup device of the CCD type shown in FIG. 10.
The image pickup device of FIG. 10 is a frame transfer type CCD. First, the information charges which were photoelectrically converted by an image pickup section 1 in correspondence to each color filter of a stripe filter are transferred at a high speed to a memory section 2 by drive pulses .phi.PI and .phi.PS for a vertical blanking period synchronized with a television. The information charges accumulated in the memory section 2 are transferred in a manner such that the information corresponding to each stripe filter is distributed and transferred to horizontal shift registers SR.sub.1, SR.sub.2, and SR.sub.3 each time the information of one horizontal line is vertically transferred. Namely, as shown in FIG. 11, in the conventional example, the information of one horizontal line of the memory section 2 is distributed to shift registers SR.sub.1 to SR.sub.3 for every color information, respectively. R, G, and B signals are output from the horizontal shift registers SR.sub.1, SR.sub.2, SR.sub.3, respectively. Therefore, the registers SR.sub.1 to SR.sub.3 constitute separating means for separating the chrominance signals.
FIG. 12 is a block diagram of a signal processing circuit of the signals read out of the CCD. A color filter as shown in, e.g., FIG. 11 is attached to the surface of an image pickup device 10 (e.g., CCD) driven by a clock IC 30 and a driver 20. The R, G, and B signals corresponding to the color separation filters are individually obtained as output signals of the device 10. These signals are reproduced as direct currents by a clamp circuit 40 and then supplied to an automatic gain control circuit (hereinafter, abbreviated to AGC circuit) 50 at the next stage, thereby setting the R, G, and B signals to the same level. As a clamp circuit, it is further desirable to use a feedback clamp circuit to feed back a DC potential of an input signal of a switch circuit 60 to the clamper. Output signals of the AGC circuit 50 are then supplied to the switch circuit 60 serving as sequencing means for forming a luminance signal at the next stage and to a process encoder 70 including circuit for performing ordinary signal processes such as gamma correction, white clip, and the like and converting the output signals to the NTSC signals. The operation of the switch circuit 60 will now be described with reference to FIG. 13. In the diagram, S.sub.1, S.sub.2, and S.sub.3 denote output signals of the CCD in FIG. 10. In this example, it is assumed that the drive pulses of the horizontal shift registers are three-phase drive pulses which are equivalent to the signal waveforms shown in FIG. 13.
The signals S.sub.1 to S.sub.3 are taken out by switch pulses of control signals SW-R, SW-G, and SW-B of the switch circuit. The signals taken out are added, so that a luminance signal shown at Y in the diagram is derived. Namely, the same signal Y as the spatial sampling of the color separation filters is obtained, so that the resolution is fairly improved. In this manner, when only the portions necessary as a luminance signal are taken out by means of the switching and added and the luminance signal is generated, no noise will be added and the S/N ratio will not be deteriorated.
In the foregoing conventional example, as shown in FIG. 10, the delay characteristic and frequency characteristic in the clamp circuit 40 and AGC circuit 50 are extremely important for the output signals S.sub.1 to S.sub.3 of three systems of the CCD.
In other words, according to the experiments, the delay time must be set to a time within .+-.20 nsec and the cut-off frequency must be set to a frequency above 10 MHz.
However, those AFC circuits generally have the drawback that the delay characteristic and frequency characteristic are extremely bad.
Thus, this drawback causes another drawback, that the MTF characteristic deteriorates and the resolution decreases.
On the contrary, there is also the drawback that in order to improve the delay characteristic and frequency characteristic of the AGC circuit, circuit current must be considerably increased and a complicated IC circuit must be to constructed using special IC processes.
On the other hand, there is also the problem that in the case where the AGC circuit is provided in the processing circuit, e.g., after the gamma correction circuit, the fluctuation of the DC component which is produced by the AGC circuit will cause a clip level error in the white clip or dark clip.